I am interested in what kind of circuit would reliably detect when a cell phone gets a call. The same concept as is used for those vibrating belt clips that were popular before phones had built in vibrators.

Can anybody point me in the right direction? I know a little about circuits, but not an EE or tech. Thanks.

Originally posted by walkonfire@Apr 5 2006, 06:46 PMI am interested in what kind of circuit would reliably detect when a cell phone gets a call. The same concept as is used for those vibrating belt clips that were popular before phones had built in vibrators.

Can anybody point me in the right direction? I know a little about circuits, but not an EE or tech. Thanks.

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It is not really clear what you are asking for. The cellphone itself will detect a call. Most of them will indicate it in a variety of ways. Do you want something connected to the phone? Do you want something which communicates with the phone wirelessly? Can you be more specific?

Originally posted by walkonfire@Apr 5 2006, 07:14 PMI want to be able to switch on a DC battery operated device when the phone rings.

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Let me try one more time. We need an idea of what you think can be detected. We need to know if a "wire" connection to the phone is possible. We need to know if you think a wireless solution is what you need. A cell phone is normally a closed system. The people who make it don't want you messing around with it. It probably does not have a little accessory port on it that you can connect to, so you can ask it things or it can tell you things. If you are willing to take it apart and hack the internals then have at it. If you want something to detect what the phone is doing without a wire connection, then life gets more difficult.

Maybe we should start with the phone. Can you get any technical information it?

Well, this would be a wireless connection, that would either detect a change in magnetic field, RF or whatever else could be detected in the proximity of the phone by virtue of the fact that it 'woke up' to receive the phone call.

I've been searching all over to see how the old-fashion vibrating clips worked, to no avail. But, what I do know is that when the phone gets a call, it switches on a small AAA-cell powered DC motor with eccentric mass to vibrate on the belt. These can be bought for $15. I may just buy one and tap into the switch/relay, unless I can have some fun making my own.

I hate to be stupid, but I'm no circuit guy. I just know that somehow these old vibrators picked up on some transient phenomenon that occurrs when the phone gets a call. My guess is its a change in inductance, reluctance, RF or magnetic field.

One last note, the vibrating belt clips out there are universal, meaning they'll work with virtually any cell phone. I just need to know how they work. All I want to do is use the cell phone to switch on a small battery operated circuit (two AA batteries) wirelessly, and NO, it's not any sort of weapon.

Originally posted by walkonfire@Apr 5 2006, 08:10 PMOne last note, the vibrating belt clips out there are universal, meaning they'll work with virtually any cell phone. I just need to know how they work. All I want to do is use the cell phone to switch on a small battery operated circuit (two AA batteries) wirelessly, and NO, it's not any sort of weapon.

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I suppose if you cloned your phone and it could receive the same signals but without the keypad an screen then you might have something. Siemmens makes several modules that are the guts of a cell phone. They may or may not require a SIM card to receive incoming calls. The modules have a UART compatible serial port and are programmed like an old Hayes modem with an AT command set. The cost is over $100.00 so this will be a pricey experiment.

...You know those cheapo, $10, off-the-shelf, universal vibrating belt clips that you can buy for any ordinary cell phone, and not only can you clip your phone to your belt with it, but it (the clip, not the phone) vibrates whenever you get a call....??

... how do those things (the clip, not the phone) know when to start vibrating, without actually being wired to the phone??

I'm pretty sure there's no telephone parts inside the clip... the darned thing just seems to know when somebody is calling. Who knows...maybe it's magic. I'm just a car mechanic.

Originally posted by walkonfire@Apr 5 2006, 05:38 PMI hate to be stupid, but I'm no circuit guy. I just know that somehow these old vibrators picked up on some transient phenomenon that occurrs when the phone gets a call. My guess is its a change in inductance, reluctance, RF or magnetic field.

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Is the phone GSM or CDMA? GSM phones can easily induce currents in nearby speakers and electronics.

...You know those cheapo, $10, off-the-shelf, universal vibrating belt clips that you can buy for any ordinary cell phone, and not only can you clip your phone to your belt with it, but it (the clip, not the phone) vibrates whenever you get a call....??

... how do those things (the clip, not the phone) know when to start vibrating, without actually being wired to the phone??

I'm pretty sure there's no telephone parts inside the clip... the darned thing just seems to know when somebody is calling. Who knows...maybe it's magic. I'm just a car mechanic.

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Sorry, I've never seen one of those devices. I couldn't tell you how they work but inductive coupling sounds plausible. I guess you could do an experiment to see what happens. With a coil of wire positioned in proximity to the phone see if your scope can measure some kind of signal above the noise floor. If you can find something non-random them we might have something to work with.

BTW if what you want costs $10 you should be happy to hack one or at least pull it apart to find out what's inside.

Your cell phone searchs for towers when it is on (not continuosly but often) It sends a signal. When someone calls your cell phone the last tower that recieved a signal sends a signal to see if your phone is still in it's cell area. The phone signals back and the call is put through. This signal is picked up by the vibrating device and switches on the vibrations. I'm not sure if it's the frequency range or the small signal and relevant proximity.

I have to agree with papabravo in that your best direction would be to purchase one of these $10 devices and hack it. I believe it has everything that you are wanting built into it. Instead of the motor that vibrates you could wire the device that you want to run.

Originally posted by dtiger2k@Apr 6 2006, 02:30 AMYour cell phone searchs for towers when it is on (not continuosly but often) It sends a signal. When someone calls your cell phone the last tower that recieved a signal sends a signal to see if your phone is still in it's cell area. The phone signals back and the call is put through. This signal is picked up by the vibrating device and switches on the vibrations. I'm not sure if it's the frequency range or the small signal and relevant proximity.

I have to agree with papabravo in that your best direction would be to purchase one of these $10 devices and hack it. I believe it has everything that you are wanting built into it. Instead of the motor that vibrates you could wire the device that you want to run.

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The frequencies of these signal are in the 1-2 GHz. range. RF design at these frequencies takes experience, equipment, and knowhow well beyond the reach of the average experimenter.

Originally posted by walkonfire@Apr 7 2006, 01:17 AMThanks guys for trying, anyway. I figured the off-the-shelf option was where I'd end up at. I'll let you know what works out.

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WHOAH - slow down - i had a similar thing which flashed when a call came (currently destroyed). on close examination, it had, what appeared to be, a TINY transformer that i think picked up the electromagnetic feild of a call. I never got the chance to try it on my electric toothbrush, but i think its true. I duno - try dismantling one anyway to make sure

Hello i read in Nuts n Volts mag a circuit a guy built ..so his hearing impaired wife could tell if the cell phone rang without having it cliped to her ..anyways the cell phone does call out before it recieves a call to let the tower know its available where its at etc etc.. the circuit he built turned a light on in the house to let her know a call was about to come in .. i think it used rf sensing to a relay so you would be able to turn dc or ac on or off, i dont know if this will help the orig poster here ..but i have all the back issues and will search for it today and post the circuit
sorry if im off topic here ...thanks Dan